/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.hadoop.hbase.client;

import java.io.Closeable;
import java.io.IOException;
import java.io.UncheckedIOException;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

import org.apache.yetus.audience.InterfaceAudience;
import org.apache.hadoop.hbase.client.metrics.ScanMetrics;

/**
 * Interface for client-side scanning. Go to {@link Table} to obtain instances.
 */
@InterfaceAudience.Public
public interface ResultScanner extends Closeable, Iterable<Result> {

    @Override
    default Iterator<Result> iterator() {
        return new Iterator<Result>() {
            // The next RowResult, possibly pre-read
            Result next = null;

            // return true if there is another item pending, false if there isn't.
            // this method is where the actual advancing takes place, but you need
            // to call next() to consume it. hasNext() will only advance if there
            // isn't a pending next().
            @Override
            public boolean hasNext() {
                if(next != null) {
                    return true;
                }
                try {
                    return (next = ResultScanner.this.next()) != null;
                } catch(IOException e) {
                    throw new UncheckedIOException(e);
                }
            }

            // get the pending next item and advance the iterator. returns null if
            // there is no next item.
            @Override
            public Result next() {
                // since hasNext() does the real advancing, we call this to determine
                // if there is a next before proceeding.
                if(!hasNext()) {
                    return null;
                }

                // if we get to here, then hasNext() has given us an item to return.
                // we want to return the item and then null out the next pointer, so
                // we use a temporary variable.
                Result temp = next;
                next = null;
                return temp;
            }
        };
    }

    /**
     * Grab the next row's worth of values. The scanner will return a Result.
     * @return Result object if there is another row, null if the scanner is exhausted.
     * @throws IOException e
     */
    Result next() throws IOException;

    /**
     * Get nbRows rows. How many RPCs are made is determined by the {@link Scan#setCaching(int)}
     * setting (or hbase.client.scanner.caching in hbase-site.xml).
     * @param nbRows number of rows to return
     * @return Between zero and nbRows rowResults. Scan is done if returned array is of zero-length
     *         (We never return null).
     * @throws IOException
     */
    default Result[] next(int nbRows) throws IOException {
        List<Result> resultSets = new ArrayList<>(nbRows);
        for(int i = 0; i < nbRows; i++) {
            Result next = next();
            if(next != null) {
                resultSets.add(next);
            } else {
                break;
            }
        }
        return resultSets.toArray(new Result[0]);
    }

    /**
     * Closes the scanner and releases any resources it has allocated
     */
    @Override
    void close();

    /**
     * Allow the client to renew the scanner's lease on the server.
     * @return true if the lease was successfully renewed, false otherwise.
     */
    boolean renewLease();

    /**
     * @return the scan metrics, or {@code null} if we do not enable metrics.
     */
    ScanMetrics getScanMetrics();
}
